The invention relates to an impact absorber of the type comprising two aligned parts of tubular overall shape and different cross sections, joined together by a joining zone:
that part of the tube which has the cross section of smaller surface area forming a plunger tube,
that part of the tube which has the cross section of larger surface area forming an absorber tube as the casing of this tube is turned back under the action of the plunger tube at the region of the join, with free sliding about said plunger tube, said turning-back beginning at the region of the joining zone.
An absorber of this type is described, in particular, in Patent Applications EP 0 763 448 (Hoogovens) and FR 2,698,674 (GKN-LOHR); these types of absorbers are, in particular, fitted to motor vehicles to deaden accidental impacts of these vehicles, under damping and deceleration conditions that can be withstood, particularly by the passengers.
An important advantage of the absorbers described in these documents lies in their compactness: the energy absorbed per unit distance of deformation is very high, commonly of the order of 1000 J for 50 mm of deformation.
The drawbacks of the absorbers described in these documents relate to:
the risk that the turning-back may begin outside the join, for example at the other end of the absorber tube;
the fact that the force required to begin the turning-back is very much higher than the force required to sustain the turning-back, resulting in an initial force xe2x80x9cspikexe2x80x9d at the start of the impact, which leads to an excessively abrupt and/or dangerous deceleration prejudicial to good damping;
the risk that the sliding of the absorber tube with its turned-back part in the plunger tube will seize and jam.
Furthermore, the methods for manufacturing absorbers of this type also exhibit drawbacks:
these methods are generally expensive, for example because of the production of the join;
starting from a steel tube, a conventional method consists in expanding the tube over that part of its length which corresponds, to the absorber tube and this expansion is generally performed by deep drawing; this method therefore entails the use of grades of steel which are suited to deep drawing, generally having a yield stress Re less than 300 MPa, this restricting the performance of the absorber.
The object of the invention is to overcome these drawbacks.
To this end, the subject of the invention is an impact absorber of the type comprising two aligned parts of tubular overall shape and different cross sections, joined together by a joining zone:
that part of the tube which has the cross section of smaller surface area forming a plunger tube,
that part of the tube which has the cross section of larger surface area forming an absorber tube as the casing of this tube is turned back under the action of the plunger tube at the region of the join, with free sliding about said plunger tube, said turning-back beginning at the region of the joining zone,
wherein, in any plane of section passing through the axis of the tube formed by these two aligned parts,
if Dp denotes the outside diameter of said plunger tube and Da denotes the outside diameter of said absorber tube,
if Ep denotes the thickness of the plunger tube and Ea denotes the thickness of the absorber tube,
if Rr.p is the natural outside radius of turning-back of the tubular casing of the plunger tube and if Rr.a is the natural outside radius of turning-back of the tubular casing of the absorber tube,
if said joining zone connects inwardly with said plunger tube, forming an outside radius of curvature Rp and connects outwardly with said absorber tube, forming an outside radius of curvature Ra,
then the following relationships are simultaneously satisfied:
Ep greater than Ea; Rp less than Rr.p; Ra greater than Rr.a; xc2xd(Daxe2x88x92Dp) greater than 2xc3x97Rr.a; Ra greater than Rp.
The invention may also exhibit one or more of the following features:
the two parts of tubular overall shape and the zone in which they join are essentially made of a metallic material, which preferably has a yield stress Re greater than 300 MPa, which may be a carbon steel or a stainless steel.
Rpxe2x89xa63xc3x97Ep.
Raxe2x89xa75xc3x97Ea.
xc2xd(Daxe2x88x92Dp) greater than Ra+xc2xdRp
xc2xd(Daxe2x88x92Dp) and (Rp+Ra) and Ra≈2xc3x97Rr.a.
Daxe2x89xa61.4 Dp.
said plunger tube has ribs.
Another subject of the invention is a method for manufacturing an impact absorber according to the invention from a tube made of a metallic material, which comprises a stage in which the cross section of this tube is expanded by hydroforming over at least part of its length so as to obtain, over at least part of the length of the tube, on one hand, that part of the tube which forms a plunger tube and, on the other hand, the expanded part which forms an absorber tube, these two parts being joined together by said joining zone.
The invention may also exhibit one or more of the following features:
the cross section of this tube is expanded over several unconnected parts along its length.
said metallic material has a yield stress Re greater than 300 MPa, and may, in particular, be a carbon steel or a stainless steel.